It is common in elevators to define a terminal stopping zone as a distance within which, if the elevator has not decelerated sufficiently to indicate that the control system is capable of stopping the elevator, the elevator should be stopped by some emergency means. The presence of the elevator within the emergency terminal zone is typically indicated by means of cams mounted in the elevator hoistway at appropriate positions so as to close switches mounted on the elevator (or vice versa). It is also known that the measurement of speed for determining whether or not emergency terminal stopping should be invoked is typically accomplished by means of a centrifugal switch on the elevator tachometer which provides a signal whenever speed is in excess of a threshold magnitude determined to be indicative of the fact that the elevator is still properly responding to a properly operating controller. Such a speed may, for instance, be 94% of rated contract speed for the given installation. In such a case, if immediately upon entering the emergency terminal zone the elevator speed has not reduced to below 94% of rated speed, then the main operating relay, which provides power to the elevator brake lifting mechanism and the main elevator motor through the safety chain, is disenergized. In addition, most elevators include mechanical safeties, particularly those that operate whenever the elevator overspeeds (such as more than 115% of rated speed).
Elevator code requirements (specified by various governments), such as American National Standard Institute A17.1, Rule 209.2b, require that emergency terminal stopping devices be so designed and installed that a single short circuit caused by a combination of grounds or by other conditions shall not prevent them from functioning. Historically, this requirement has been satisfied through the use of redundant stopping switch contacts above and below the main relay in the safety chain, as well as the use of redundant speed contacts on the governor.
However, even the redundancy does not provide absolute safety because it is possible for contacts to become welded together, or other circuit failures to occur that do not manifest themselves during normal operation, and therefore the failure of which remains unknown. Additionally, the use of stopping switches on an elevator is expensive and difficult to implement in an elevator shaft. The cost of providing safe, emergency terminal stopping is also a consideration.